Question

The correct representation of a complex ion is:

• A. \([Co(H_2O)(NH_3)_4Cl]^{2+}\)
• B. \([CoCl(H_2O)(NH_3)_4]^{2+}\)
• C. \([Co(NH_3)_4Cl(H_2O)]^{2+}\)
• D. \([Co(NH_3)_4(H_2O)Cl]^{2+}\)

Hint:

Write ligands alphabetically by their names: ammine (NH\(_3\)), then aqua (H\(_2\)O), then chloro (Cl\(^-\)).

Answer 1

The Correct Option is D

Concept: In naming coordination complexes (and writing their formulas), ligands are arranged in alphabetical order according to their names (ignoring prefixes like di-, tri-, tetra-).

Step 1: Identify the ligands and their names.
• \(NH_3\) → ammine (starts with 'a')
• \(H_2O\) → aqua (starts with 'a' as well — compare: 'a' of ammine vs 'a' of aqua → then compare second letter: m vs q → 'a' then 'm' comes before 'q'? Actually alphabetical: 'a' then next letter. 'ammine' (a m m i n e) vs 'aqua' (a q u a). 'm' comes before 'q', so ammine comes before aqua.)
• \(Cl^-\) → chloro (starts with 'c', which comes after 'a')

Step 2: Correct alphabetical order. \[ \text{ammine (NH}_3)<\text{aqua (H}_2O)<\text{chloro (Cl}^-) \] However, both ammine and aqua start with 'a'. Compare second letters: 'm' (ammine) vs 'q' (aqua). Since 'm' comes before 'q', ammine comes before aqua. Thus, the correct sequence is: \(NH_3\) first, then \(H_2O\), then \(Cl\).

Step 3: Write the formula. \[ [Co(NH_3)_4(H_2O)Cl]^{2+} \] This matches option (D).

Step 4: Why not other options?
• (A) and (B) do not follow alphabetical order.
• (C) has \(Cl\) before \(H_2O\), which is incorrect because 'aqua' (a) comes before 'chloro' (c).